Protonic defects in hydrogen bonded liquids: Structure and dynamics in ammonia and comparison with water

The structural, dynamical, and electronic properties of ionic defects in liquid ammonia at 260 K created by the addition or removal of a proton have been studied using the method of ab initio molecular dynamics. These protonic defects correspond to the ammonium(NH₄⁺) and amide(NH₂^-) ions in the liquid and are the analogues of the H₃O⁺ and OH^- ions in water. For this reason, direct comparison between the protonic defects in ammonia and those in water can be made. In particular, it is found that the NH₄⁺ exhibits a characteristic cationic solvation pattern, in which it donates four hydrogen bonds to neighboring ammonia molecules, giving it a coordination number of 4. The NH₂^- ion is found to have a coordination number between 7 and 8 in liquid ammonia, a number higher than would be expected based on the number of hydrogen bonds it can accept and donate. It is found that about 40% of this is due to hydrogen bonding but that these hydrogen bonds are all accepted by the amide nitrogen. Moreover, the hydrogen bonds are often arranged in a planar configuration(perpendicular to the C₂ axis of the amide), a solvation pattern also exhibited by OH in water. The rationale for the high coordination of NH₂^- is found to differ markedly from that which emerges from interpretation of spectral data. Unlike H₃O⁺ and OH^- in water, no proton transfer is exhibited in either the NH₄⁺ system or the NH₂^- system. The results presented here lead to a possible explanation for the lack of structural diffusion. Nevertheless, the solvation structures formed by the NH₄⁺ and NH₂^- ions in ammonia and their associated electronic properties possess many similarities with the water ions in water, and from the studies performed here, a number of important patterns begin to emerge that may be applicable to protonic defects in other hydrogen-bonded liquids.